Controlling circuit for electric shutters

ABSTRACT

A controlling circuit for electric shutters comprising at least one constant current circuit, a pair of capacitors to be charged in advance by a current source battery through the constant current circuit and a pair of driving coils to be energized in turn by discharge currents of the pair of capacitors to start a shutter blade opening motion and closing motion so that the current source battery of a comparatively small capacity may be effectively used.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to controlling circuits for electric shutters and more particularly to improvements in an electric circuit wherein a shutter blade opening motion and closing motion are made to be started respectively by a pair of electromagnets to be energized by discharge currents of capacitors.

(B) Description of the Prior Art

A conventional example of this kind of controlling circuit is shown in FIG. 1 in which symbol E₀ indicates a current source battery, S₀ indicates a current source switch, L₁ indicates a driving coil serving to start a shutter blade opening motion, L₂ indicates a driving coil serving to start a shutter blade closing motion, C₁ and C₂ indicate capacitors for respectively energizing the driving coils L₁ and L₂, R₁ and R₂ indicate resistors for respectively charging the capacitors C₁ and C₂, D₁ and D₂ indicate diodes for respectively preventing reverse flows, T₁ and T₂ indicate transistors to be respectively used as switching elements for connecting the coils L₁ and L₂ respectively to the capacitors C₁ and C₂, Rv and C₃ indicate respectively a variable resistor and a capacitor forming a CR delay circuit for controlling the exposure time and S₁ indicates a switch for starting the operation.

This circuit operates as follows. First of all, when the current source switch S₀ is closed, the capacitors C₁ and C₂ will be charged by the current source battery E₀ respectively through the diode D₁ and resistor R₁ and through the diode D₂ and resistor R₂. Then, when the switch S₁ is closed, the transistor T₁ will be on, a large electric current will be instantaneously fed to the driving coil L₁ from the capacitor C₁, a locking lever not illustrated will be moved by the energization of the driving coil L₁ and a shutter blade opening motion will be started. In such case, the current from the capacitor C₁ will be checked by the diode D₁ and therefore will not flow into the driving coil L₂. On the other hand, by the closing of the switch S₁, the transistor T₃ will be on, the transistor T₄ will be off and therefore charging the capacitor C₃ through the variable resistor Rv will be started. When a certain time, that is, a proper exposure time elapses, both transistors T₅ and T₆ will be on and the transistor T₂ will be on. Thereby, a large current will be instantaneously fed to the driving coil L₂ from the capacitor C₂ and the driving coil L₂ will be energized. By this energization of the driving coil L₂, a locking lever not illustrated will be moved and a shutter blade closing motion will be started. In such case, the current from the capacitor C₂ will be checked by the diode D₂ and therefore will not flow into the driving coil L₁.

According to the above mentioned conventional circuit, in case the capacitance of the capacitors C₁ and C₂ is C, the voltage of the current source battery E₀ is Vcc, the charge current is I and the voltage between the terminals of the capacitors C₁ and C₂ is Vc, the time t until charging the capacitors C₁ and C₂ is completed will be represented by the formula ##EQU1## and the charge current I will be represented by the formula ##EQU2## As evident from the above mentioned formulas (1) and (2) and the characteristic curve (a) in FIG. 3, in the case of such CR charging system as in the conventional circuit, a comparatively long time will be required from the starting of charging the capacitors C₁ and C₂ until the voltage Vc between their terminals reaches a predetermined valve. Therefore, in the case of using this circuit, the shutter chance will be likely to be missed. In order to shorten this charging time, the capacity of the current source battery may be enlarged so that the maximum output current (charge current) may be increased (see the characteristic curve (c) in FIG. 3). However, in the case by such process, a disadvantage that the current source battery will have to be large will occur.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a controlling circuit for electric shutters of the above mentioned type wherein capacitors are made chargeable with a constant current to eliminate the above mentioned defects.

Another object of the present invention is to provide a controlling circuit for electric shutters of the above mentioned type wherein a pair of capacitors are made chargeable through one constant current circuit to simplify the circuit formation.

A further object of the present invention is to provide a controlling circuit for electric shutters of the above mentioned type wherein, to simplify the circuit formation, constant current circuit elements are so connected that, when a discharge current is made to flow from a capacitor to one driving coil, the discharge current may not flow into the other driving coil.

These and other objects of the present invention will become more apparent during the course of following detailed description and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a wiring diagram showing a conventional example of an electric shutter controlling circuit related to the present invention;

FIG. 2 is a wiring diagram showing an embodiment of an electric shutter controlling circuit according to the present invention;

FIG. 3 is a diagram for explaining the charging characteristics of a capacitor; and

FIG. 4 is a wiring diagram showing another embodiment of an electric shutter controlling circuit according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows an embodiment of an electric shutter controlling circuit according to the present invention. This embodiment is fundamentally the same as the circuit shown in FIG. 1 except that a pair of capacitors C₁ and C₂ are respectively made to be charged through one constant current circuit.

Therefore, in this embodiment, the same corresponding symbols are used for the same elements as in FIG. 1 and the explanation of the general operation is omitted.

Referring to FIG. 2, a constant current circuit consists of a transistor T₇ whose base is connected to a negative pole of a current source battery E₀ through a resistor R₃ and whose emitter is connected to a positive pole of the battery E₀ through a current source switch S₀. The output side of the constant current circuit, that is, the collector of the transistor T₇ is connected to a pair of capacitors C₁ and C₂ respectively through diodes D₁ and D₂. Therefore, according to this embodiment, the time t until charging the capacitors C₁ and C₂ is completed after the current source switch S₀ is closed is represented by the formula

    t = (Vcc.C)/I                                              (3),

the charge current I is represented by the formula

    I = (Vcc.C)/t                                              (4)

and the charging characteristics for the capacitors C₁ and C₂ are as shown by a straight line (b) in FIG. 3.

Now, in case the voltage Vcc of the current source battery E₀ is 24 V, the capacitance value C of the capacitors C₁ and C₂ is 1000 μF and the charge current (the maximum output current of the current source battery E₀) value I is 50 mA, according to such CR charging system as in the circuit of FIG. 1, as evident from the above mentioned formula (1), the charging time t required for the voltage Vc between the terminals of the capacitors C₁ and C₂ to reach a value of 95% of the current source voltage Vcc will be about 1.5 second (see the curve (a) in FIG. 3). On the other hand, according to such constant current charging system as in the circuit of FIG. 2, as evident from the above mentioned formula (3), the charging time t required for the voltage Vc between the terminals of the capacitors C₁ and C₂ to reach the value of the current source voltage Vcc may be about 0.5 second (see the curve (b) in FIG. 3). By the way, in order that the time until the voltage Vc between the terminals of the capacitors C₁ and C₂ reaches a value of 95% of the current source voltage Vcc by such CR charging system as in the circuit of FIG. 1, that is, the charging time t may be 0.5 second (see the curve (c) in FIG. 3), as evident from the above mentioned formula (2), the current source battery E₀ whose maximum output current I is 150 mA will be required.

As evident from the above explanation, according to such constant current charging system as in the present invention, in case a current source battery of the same capacity is used, the time required to charge the capacitors to a predetermined valve will be reduced to be 1/3 the time in the case of such CR charging system as in the conventional circuit. Also, in order that the time required until the completion of the chargeing may be the same, in the case of the constant current chargeing system, the current source capacity may by 1/3 that in the case of the CR charging system. This means that the circuit of FIG. 2 is very favorable as a controlling circuit for electric shutters.

FIG. 4 shows another embodiment of the present invention. This embodiment is different from the embodiment in FIG. 2 in respect that the constant current circuit to be utilized to charge the capacitor C₁ consists of the resistor R₃ and transistor T₇, the constant current circuit to be utilized to charge the capacitor C₂ consists of the resistor R₄ and transistor T₈, and the transistors T₇ and T₈ are to perform the same roles as of the diodes D₁ and D₂. That is to say, in this embodiment, the constant current circuits for the capacitors C₁ and C₂ are provided independently of each other so that, when the capacitor C₁ is discharged, its discharge current may be prevented by the transistor T₇ from flowing into the driving coil L₂ and, when the capacitor C₂ is discharged, its discharge current may be prevented by the transistor T₈ from flowing into the driving coil L₁. As the fundamental formation and operation of this embodiment are also the same as of FIG. 2, the same corresponding symbols are attached to the same or similar elements and the explanation of the operation shall be omitted. 

I claim:
 1. A controlling circuit for electric shutters comprising a first capacitor, a first driving coil connected to said first capacitor, a first switching means connected between said first capacitor and first driving coil and capable of flowing a discharge current to said first driving coil from said first capacitor when made to conduct to open a shutter, an exposure time controlling circuit connected to said first switching means, a second switching means connected to an output terminal of said exposure time controlling circuit and made to conduct by an electric current issued from said exposure time controlling circuit, a second driving coil connected to said second switching means and working to close the shutter when energized, a second capacitor connected between said second switching means and second driving coil and flowing a discharge current to said second driving coil to energize it when said second switching means is made to conduct, and a constant current circuit connected to said first and second capacitors to charge them.
 2. A controlling circuit for electric shutters according to claim 1 wherein said constant current circuit consists of a transistor having a base connected to one terminal of each of said first and second capacitors and an collector connected to the other terminal of each of said first and second capacitors respectively through diodes, said diodes being made to serve to prevent a discharge current from said first capacitor from flowing into said second driving coil and to prevent a discharge current from said second capacitor from flowing into said first driving coil.
 3. A controlling circuit for electric shutters according to claim 2 wherein a resistor is connected between the base of said transistor and said one terminal of each of said first and second capacitors.
 4. A controlling circuit for electric shutters according to claim 1 wherein said constant current circuit consists of a first transistor having a base connected to one terminal of said first capacitor and a collector connected to the other terminal of said first capacitor and a second transistor having a base connected to one terminal of said second capacitor and a collector connected to the other terminal of said second capacitor, said first transistor being made to serve to prevent a discharge current from said first capacitor from flowing into said second driving coil, and said second transistor being made to serve to prevent a discharge current from said second capacitor from flowing into said first driving coil.
 5. A controlling circuit for electric shutters according to claim 4 wherein resistors are connected respectively between the base of said first transistor and said one terminal of said first capacitor and between the base of said second transistor and said one terminal of said second capacitor. 